1. Field of the Invention
This invention relates generally to composite threaded fasteners, and more particularly to a high temperature-resistive metallic threaded composite fastener having a ceramic coated body with a non-circular shank and an externally threaded split collar of metal alloy which is secured to the shank of the body by a lock ring.
2. Brief Description of the Prior Art
Current state of the art hypervelocity vehicle thermal protection systems embodied in the Space Shuttle employ the use of carbon composite panels for protection in the highest temperature regions and glass coated sintered ceramic fiber tiles bonded onto the substructure for protection in the lower temperature regions. The carbon composite panels typically incorporate ninety degree (90.degree.) flanged edges which are oriented toward the substructure and which are attached to the substructure with metallic fittings and fasteners.
The above described current thermal protection system has several disadvantages. The glass coated sintered ceramic fiber tiles are weak, easily damaged, and difficult to install and remove. Access must be provided to the region of the mechanical attachment hardware under the surface of the carbon composite panels and tiles. Seals are required for the gaps between the panels and the tiles. Thus, the current Shuttle thermal protection system is labor intensive, and therefore expensive to fabricate, install, and maintain. Although oxidation-resistant coatings are used in high temperature application, they are very brittle, weak in tension and shear, and cannot be threaded.
It would therefore be desirable to provide a fastener which can tolerate very high temperatures and would allow the development of a simplified thermal protection system.
There are several patents which disclose various composite fasteners, attachable threaded collars, and two part threaded members.
Bjork, U.S. Pat. No. 3,159,075 discloses a two-part fastener secured by a snap ring, however, it uses rubber O-rings and its metallic parts are exposed to the surface on which it is mounted.
Coel et al, U.S. Pat. No. 4,126,338 discloses an externally threaded collar attachable to a primary member, but would not be suitable for use with high temperature composite materials because its axial constraint means uses small sharp edges which would overstress relatively weak and brittle composite materials.
White, U.S. Pat. No. 4,486,134 discloses a complicated wood screw having an externally threaded collar attached by means of a snap ring.
Berecz, U.S. Pat. No. 4,718,801 discloses a composite threaded fastener but would not be suitable for high temperature applications because the metallic portion lies too close to the surface on which it is mounted and the thermal expansion differences between the metallic and composite portions would overstress the parts.
Dunsmore, U.S. Pat. No. 4,861,211, discloses a composite threaded fastener but is only applicable to plastic materials which would be limited to 350.degree. F.
Olez et al, U.S. Pat. No. 4,863,330 disclose a composite threaded fastener made entirely of low-temperature plastics.
The present invention is distinguished over the prior art in general, and these patents in particular by a metallic threaded composite fastener, particularly suited for high temperature applications, which has a body member made of high temperature-resistant composite material with a ceramic coating. The body member has a head portion configured to be installed in a countersunk hole and a shank portion which is non-circular and tapered. One part of the shank may be non-circular and the other part tapered, or the two types of surfaces could be combined into a frustum of a non-circular cone. A split collar member made of high strength, high temperature-tolerant metal alloy is split into two halves and the interior of the halves are configured to engage the shank. The exterior of the collar has external threads and the upper portion of the collar has a circumferential groove which receives a lock ring to secure the collar halves to the shank. In the assembled condition torque may be transmitted from the body to the split collar by the engaged non-circular portions to install and remove the fastener assembly into or from a threaded aperture and shear loads in the collar threads are transferred to the shank tapered portion as a combination of radial compression and axial tension loads. Thus, tension loads may be applied to the fastener shank without damaging the ceramic coating.